1、编号： 毕业设计(论文)外文翻译（原文）学 院： 专 业： 学生姓名： 学 号： 指导教师单位： 姓 名： 职 称： *年 * 月 * 日The technology of Microlens array injection moldingAbstract Injection molding could be used as a mass production technology for microlens arrays. It is of importance, and thus of our concern in the present study, to understand the
2、injection molding processing condition effects on the replicability of microlens array profile. Extensive experiments were performed by varyingprocessing conditions such as flow rate, packing pressure and packing time for three different polymeric materials (PS, PMMA and PC). The nickel mold insert
3、of microlens arrays was made by electroplating a microstructure master fabricated by a modified LIGA process. Effects of processing conditions on the replicability were investigated with the help of the surface profile measurements. Experimental results showed that a packing pressure and a flow rate
4、 significantly affects a final surface profile of the injection molded product. Atomic force microscope measurement indicated that the averaged surface roughness value of injection molded microlens arrays is smaller than that of mold insert and is comparable with that of fine optical components in p
5、ractical use.1 Introduction Microoptical products such as microlenses or microlens arrays have been used widely in various fields of microoptics, optical data storages, bio-medical applications, display devices and so on. Microlenses and microlens arrays are essential elements not only for the pract
6、ical applications but also for the fundamental studies in the microoptics. There have been several fabrication methods for microlenses or microlens arryas such as a modified LIGA process 1, photoresist reflow process 2, UV laser illumination 3, etc. And the replication techniques, such as injection
7、molding, compression molding 4 and hot embossing 5, are getting more important for a mass production of microoptical products due to the cost-effectiveness. As long as the injection molding can replicate subtle microstructures well, it is surely the most cost-effective method in the mass production
8、stage due to its excellent reproducibility and productivity. In this regard, it is of utmost importance to check the injection moldability and to determine the molding processing condition window for proper injection molding of microstructures. In this study, we investigated the effects of processin
9、g conditions on the replication of microlens arrays by the injection molding. The microlens arrays were fabricated by a modified LIGA process, which was previously reported in 6, 7. Injection molding experiments were performed with an electroplated nickel mold insert so as to investigate the effects
10、 of some processing conditions. The surface profiles of molded microlens arrays were measured, and were used to analyze effects of processing conditions. Finally, a surface roughness of microlens arrays was measured by an atomic force microscope (AFM).2 Mold insert fabricationMicrolens arrays having
11、 several different diameters were fabricated on a PMMA sheet by a modified LIGA process 6. This modified LIGA process is composed of an X-ray irradiation on the PMMA sheet and a subsequent thermal treatment. The X-ray irradiation causes the decrease of molecular weight of PMMA, which in turn decreas
12、es the glass transition temperature and consequently causes a net volume increase during the thermal cycle resulting in a swollen microlens 7. The shapes of microlenses fabricated by the modified LIGA process can be predicted by a method suggested in 7.The microlens arrays used in the experiments we
13、re composed of 500m -(a 2 2 array), 300m -(2 2) and 200m (5 5) diameter arrays, and their heights were 20.81, 17.21 and 8.06 m, respectively. Using the microlens arrays fabricated by the modified LIGA process as a master, a metallic mold insert was fabricated by a nickel electroplating for the injec
14、tion molding. Typical materials used in a microfabrication process, such as silicon, photoresists or polymeric materials, cannot be directly used as the mold or the mold insert due to their weak strength or thermal properties. It is desirable to use metallic materials which have appropriate mechanic
15、al and thermal properties to endure both a high pressure and a large temperature variation during the replication process. Therefore, a metallic mold insert is being used rather than the PMMA master on silicon wafer for mass production with such replication techniques. Otherwise special techniques s
16、hould be adopted as a replication method, e.g. a low pressure injection molding 8.The size of final electroplated mold insert was 30 30 3 mm. The electroplated nickel mold insert having microlens arrays is shown in Fig. 1.Fig.1.Moldinsert fabricated by a nickel electroplating (a) Real view of the mo
17、ld insert (b) SEM image of 200 m diameter microlens array (c) SEM image of 300 mdiameter microlens array3 Injection molding experimentsA conventional injection molding machine (Allrounders 220 M, Arburg) was used in the experiments. A mold base for the injection molding was designed to fix the elect
18、roplated nickel mold insert firmly with the help of a frametype bolster plate (Fig. 2). Shape of aperture of the bolster plate (in this study, a rectangular one) defines the outer geometry of the molded part on which the profiles of microlens arrays are to be transcribed. The mold base itself has de
19、livery systems such as sprue, runner and gate which lead the molten polymer to the cavity formed by the bolster plate, the mold insert and amoving mold surface. The mold base was designed such that mold insert replacement is simple and easy. Of course, one may introduce an appropriate bolster plate
20、with a specific aperture shape. Fig. 2. Mold base and mold insert used in the injection molding experimentThe injection molding experiments were carried out with three general polymeric materials PS (615APR, Dow Chemical), PMMA (IF870, LG MMA) and PC (Lexan 141R, GE Plastics). These materials are qu
21、ite commonly used for optical applications. They have different refractive indices (1.600, 1.490 and 1.586 for PS, PMMA and PC, respectively), giving rise to different optical properties in final products, e.g. different foci with the same geometry. The injectionmolding experiments were performed fo
22、r seven processing conditions by changing flow rate, packing pressure and packing time for each polymeric material. Furthermore, same experiments were repeated three times for checking the reproducibility. It may be mentioned that the mold temperature effect was not considered in this study since th
23、e temperature effect is relatively less important for these microlens arrays due to their large radius of curvature than other microstructures of high aspect ratio. For high aspect ratio microstructures, we are currently investigating the temperature effect more closely and plan to report separately
24、 in the future. Therefore, flow rate, packing pressure and packing time were varied to investigate their effects more thoroughly with the mold temperature unchanged in this study. Table 1 shows the detailed processing conditions for three polymeric materials. Other processing conditions were kept un
25、changed during the experiment. The mold temperatures were set to 80, 70 and 60 _C for PC, PMMA and PS, respectively.It might be mentioned that we carried out the experiments without a vacuum condition in the mold cavity considering that the large radius of curvature of the microlens arrays in the pr
26、esent study will not entrap air in the microlens cavity during the filling stage.Table 1. Detailed processing conditions used in the injection molding experimentsCaseFlow rate (cc/sec)Packing time (sec)Packing pressure(MPa)112.05.010.0212.05.015.0312.05.020.0PS412.02.010.0512.010.010.0618.05.010.072
27、4.05.010.0PMMA16.010.010.026.010.015.036.010.020.046.05.010.05676.09.012.015.010.010.010.010.010.0PC 16.05.05.026.05.010.0356.06.09.05.010.015.05.065.05.0712.05.05.04 Results and discussionBefore detailed discussion of the experimental results, it might be helpful to summarize why flow rate, packing
28、pressure and packing time (which were chosen as processing conditions to be varied in this study) affect thereplication quality. As far as the flow rate is concerned, there may exist an optimal flow rate in the sense that too small flow rate makes too much cooling before a complete filling and thus
29、possibly results in so-called short shot phenomena whereas too high flow rate increases pressure fields which is undesirable.The packing stage is generally required to compensate for the volume shrinkage of hot molten polymer whencooled down, so that enough material should flow into a mold cavity du
30、ring this stage to control the dimensionalaccuracy. The higher the packing pressure, the longer the packing time, more material tends to flow in. However, too much packing pressure sometimes may cause uneven distribution of density, thereby resulting in poor opticalquality. And too long packing time
31、 does not help at all since gate will be frozen and prevent material from flowing into the cavity. In this regard, one needs to investigate the effects of packing pressure and packing time.4.1 Surface profilesFigure 3 shows typical scanning electron microscope (SEM) images of the injection molded mi
32、crolens arrays for different diameters for PMMA (a) and different materials (b). Cross-sectional surface profiles of the mold insert and all the injection molded microlens arrays were measured by a 3D profile measuring system (NH-3N, Mitaka).(a)Injection molded microlensarrays (PMMA) (b) Injectionmo
33、lded microlenses of 300 mdiameter for different materialsFig. 3. SEM images of theinjection molded microlensarrays and microlensesAs a measure of replicability, we have defined a relative deviation of profile as the height difference between the molded one and the corresponding mold insert for each
34、microlens divided by the mold insert one. The computed relative deviations for all the microlenses are listed in Table 2.Diameter( m)Relative deviation (%)1234567PS200300500-7.625.862.38-7.592.03-0.382.082.860.51-5.565.611.47-8.6660161.47-11.444.291.47-9.475.731.95PMMA2003005007.205.77-0.661.315.60-
35、1.62-3.886.453.98-5.805.952.80-0.975.95-0.72-8.536.68-0.904.86-2.62-0.72PC20030050023.026.20-0.9316.054.965.0916.872.66-1.8619.664.531.8833.974.786.9618.671.792.43-2.944.15-1.55It may be mentioned that the moldability of polymeric materials affects the replicability. Therefore, the overall relative
36、deviation differs for three polymeric materials used in this study. It may be noted that PC is the most difficult material for injection molding amongst the three polymers. The largest relative deviation can be found in PC for the smallest diameter case, as expected. In that specific case, the large
37、st value is corresponding to the low flow rate and low packing pressure. Packing time in this case does not significantly affect the deviation. The relative deviation for PS and PMMA with the smallest diameter is far better than PC case.Table 2 indicates that the larger the diameter, the smaller the
38、 relative deviation. The larger diameter microlens is, of course, easier to be filled than smaller diameter during the filling stage and packing stage. Microlenses of larger diameters were generally replicated well regardless of processing conditions and regardless of materials. The best replicabili
39、ty is found for the case of PS with 500 m diameter. Generally, PS has a good moldability in comparison with PMMA and PC.It may be mentioned that some negative values of relative deviation were observed mostly in the smallest diameter case for PS and PMMA according to Table 2. In these cases, however
40、 the absolute deviation is an order of 0.1 m in height, which is within the measurement error of the system. Therefore, the negative values could be ignored in interpreting the experimental data of replicability. Surface profiles of microlens of 300 m diameter are shown in Figs. 4 and 5 for PC and
41、PMMA, respectively. As shown in Fig. 4, the higher packing pressure or the higher flow rate results in the better replication of microlens for the case of PC, as mentioned above. Packing time has little effect on the replication for these cases. For the case of PMMA, the packing pressure and packing
42、 time have insignificant effect as shown in Fig. 5; however, flow rate has the similar effect to PC. It might be reminded that packing time does not affect the replicability if a gate is frozen since frozen gate prevents material from flowinginto the cavity. Therefore, the effect of packing time dis
43、appears after a certain time depending on the processing conditions.Fig.4ac(leftside).Surfce profiles of microlens (PC with diameter (/) of 300 m). a effect of packing pressure, b effect of flow rate, c effectof packing timeFig.5ac.(rightside)Surface profiles of microlens (PMMA with diameter(/) of 3
44、00m). a effect of packing pressure, b effect of flow rate,c effect of packing time4.2 Surface roughnessAveraged surface roughness, Ra, values of 300 m diameter microlenses and the mold insert were measured by an atomic force microscope (Bioscope AFM, Digital Instruments). The measurements were perfo
45、rmed around the top of each microlens and the measuring area was 5 m 5 m. Figure 6 shows AFM images and measured Ra values of microlenses. PMMA replicas of microlens have the lowest Ra value, 1.606 nm. It may be noted that AFM measurement indicated that Ra value of injection molded microlens arrays
46、is smaller than the corresponding one of the mold insert. The reason for the improved surface roughness in the replicated microlens arrays is not clear at this moment, but might be attributed to the reflow caused by surface tension during a cooling process. It may be further noted that the Ra value
47、of injection molded microlens arrays is comparable with that of fine optical components in practical use.a Nickel mold insert, b PS, c PMMA, d PCFig. 6. AFM images and averaged surface roughness, Ra, values of the mold insert and injection molded 300 m diameter microlenses.4.3 Focal lengthThe focal lengt
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